Study the Impact of Intraseasonal Oscillations on Water Vapour Transport in Central Africa: The Case of 25–70 Day Oscillations

Abstract

ABSTRACT The impact of the amplitude characteristic of the intra‐seasonal oscillation (ISO) on water vapour transport is examined for the March–May (MAM) and September–November (SON) seasons. The ISO temporal indices are constructed from daily anomalies of the first two principal components of the empirical orthogonal function (EOF) applied on outgoing longwave radiation anomalies, filtered for 25–70 days. A threshold method subsequently applied to the normalised ISO amplitude using the first two EOF components is used to identify extreme events associated with ISO peaks. This allows identifying 129 (119) strong ISO events (sISOs) and 39 (40) weak ISO events (wISOs) during the MAM (SON) season. The findings revealed in MAM that, during sISO events, the southern part of the domain experiences a strengthening of the moisture convergence on the Atlantic coast, to the south‐east, associated with a westerly moisture transport. In addition, sISO induces negative anomalies in the horizontal component of moist static energy (MSE) advection. wISO are characterised by a predominance of significant moisture divergence to the east of the domain and in the centre of the Congo Basin, accompanied by moisture transport from the Indian Ocean. This event generates positive moisture advection anomalies, fuelled by a zonal influx of MSE. In contrast, the two examined ISO events exhibit opposing patterns of moisture transport during the SON season. Specifically, a dipole structure emerges with divergence anomalies in the north coinciding with convergence anomalies in the south during the sISO. Analysis of the zonal components of moisture transport indicates a reinforcement during wISO events in both seasons, due to the reinforcement of the African Easterly Jet (AEJ). During wISOs, stronger low‐level westerlies facilitate increased moisture import from the Atlantic Ocean. Furthermore, compared to sISO events, wISOs are associated with more intense northern and southern components of the AEJ. Finally, uncentred pattern correlation coefficients between strong and weak ISO events and the different phases of the Madden‐Julian Oscillation (MJO) vary depending on the season and the specific MJO phase.